This invention relates to a code-division, multiple access base station having transmit diversity.
Code-division, multiple-access (CDMA) systems have been deployed in the U.S. and elsewhere under the CDMA Telecommunications Industry Association (TIA)/Electronics Industry Association (EIA) IS-95A and IS-95B standards. Some service providers have expended considerable capital on wireless infrastructure to expand wireless coverage areas to meet subscriber demand. As new standards evolve, the older equipment in service that complies with CDMA TIA/EIA IS-95A and IS-95B is faced with potential obsolescence.
One new CDMA standard is referred to as TIA/EIA IS-2000 or IS-95C. The IS-95C standard is able to enhance system capacity in situations where mobile subscribers have low mobility by transmitting diversity signals from a base station to a mobile station. For example, under the IS-95C standard a base station may use orthogonal transmit diversity. Orthogonal transmit diversity refers to transmitting different portions of a signal from different downlink antennas to a mobile station.
Certain aspects of the IS-95C standard may be added as upgrades to the existing IS95-A or IS-95B base stations. For example, certain IS-95B digital signal processing boards may be replaced with IS-95C digital signal processing boards. However, the feature of orthogonal transmit diversity can not be added by simply upgrading boards in an existing IS-95A or IS95-B base station in the field. Accordingly, in order to provide transmit diversity, some service providers may elect to provide duplicative IS-95C base stations that overlay the coverage of existing IS-95B and IS-95A base stations. Under such circumstances, the service provider may need to provide duplicative base station antennas or even additional monopoles or towers, where tower space is unavailable. Such expenditures may ultimately discourage service providers from embracing and purchasing IS-95C equipment. Thus, a need exists for conveniently upgrading an IS-95B or IS-95A base station to operate under the IS-95C standard with transmit diversity.
In accordance with one aspect of the invention, a single base station supports the non-diversity transmission of a first digital signal type and the diversity transmission of a second digital signal type. The base station includes primary channel boards for modulating and encoding the first digital signal type and secondary channel boards for modulating and encoding the second digital signal type. A composite signal represents a combination of the first digital signal type and the second digital signal type. A first group of transmit radio modules accepts the composite digital signal from a combiner or a suitable channel board and provides a composite electromagnetic output signal. Each transmit radio module is preferably associated with a different sector. A second group of transmit radio modules accepts the second digital signal type from secondary channel boards and provides a diverse electromagnetic output signal for complementing at least a corresponding diversity component of the composite electromagnetic output signal.